Fade in an image in an animation in blender 2.9
(NOTE: You need to have the &quot;Import images to planes&quot; addon enabled to do this).
Step 1 (Import the image):

Step 2 (Open the shader editor):

Step 3 (Add a mix shader in between the principled shader and the material output (Change the factor to 0), then add a transparent shader to the bottom shader input):

Step 4 (Go to the frame you want the image to start fading out (For me it's frame 1)):

Step 5 (Add a keyframe to the factor):

Step 6 (Go to the frame you want the image to fully fade out, and change the factor value to 1):

Step 7 (Add a keyframe to the factor of 1):

And you're done!
When you render it, it should look like this:

(Sorry for the image, it was the last thing in my downloads folder).